I looked at the military pay grades and saw that the Commandant of the Coast Guard isn't like, e.g., the Chief of Staff Army; he gets the base O-10 pay. So, unlike the Chief and Vice Chief of Staff Army, the Commandant and the Vice Commandant are now equal in rank. In theory, I suppose, if the Commandant was a hotshot who got promoted "over" his soon-to-retire Vice Commandant, the Vice Commandant would have precedence because of time in grade.

(a)Allowance for Officers Serving in Certain Ranks or Positions.(5) $4,000 a year, in place of any other personal money allowance authorized by this section, while serving as Chief of Staff of the Army, Chief of Naval Operations, Chief of Staff of the Air Force, Commandant of the Marine Corps, Commandant of the Coast Guard, or Chief of the National Guard Bureau.

Edit: I think the DFAS chart you referenced was only addressing the DOD services, and not the USCG, USPHS or NOAA.